This invention relates to a semiconductor wafer loading method and apparatus. More particularly, it pertains to such a method and apparatus wherein a plurality of wafers are simultaneously transferred between a wafer transport container positioned at a delivery station and a process container disposed at a wafer-processing station.
The need for cleanliness during the manufacture of integrated circuits on semiconductor wafers is well-known. Airborne particles settle on the wafers and become integrated into the wafer through processing. This contamination of the wafer causes failure of integrated circuits manufactured on it. The continuing decrease in line circuit geometries has spurred a well publicized drive for improved cleanliness in all phases of wafer production. In terms of cleanliness, one of the most difficult items to deal with in the fabrication area is the presence of a human operator. Studies have put as many as fifty thousand particles per cubic foot (1.77 million particles per cubic meter) within one inch (2.5 cm) of an operator's skin. Any exposure of wafers to this level of contamination is certain to transfer particles to the wafer surface, resulting in substantially reduced integrated circuit yield, as discussed above.
Wafer handling for most processes involving treatment of wafers has been automated to the level of providing automated transfer of wafers one at a time out of a transport container or cassette to the process apparatus and back into a cassette at the end of the process. This reduces the need for operator handling but prolongs the treatment process due to the individual handling of the wafers. The wafers are kept under laminar flow hoods during loading and unloading operations. Processing is performed in clean chambers and exposure to an operator is minimized.
However, the handling of wafers for processing in diffusion furnaces has not followed this trend toward automation. Although the furnace, gas and time control systems are highly automated for the actual furnace operation, no practical arrangement for automatically providing cassette-to-process and back-to-cassette handling of wafers has heretofore been demonstrated. Most wafer fabrication systems use a combination of separate transfer machines in furnace loaders with a human operator exchanging the cassettes and furnace boats, which are wafer containers separable from a carrier which shifts the loaded boats in and out of the furnace. Some automated wafer handling systems exist for diffusion furnaces having separate transfer machines and furnace loaders using one or more custom-built robots to perform the movement of cassettes and boats. However, none of these machines operate continuously without human intervention. due to the fact that several machines are used to perform the various wafer-handling operations, the loading systems suffer from greatly increased complexity and a lowered mean time between machine (and therefore system) failure. Further, it is difficult to provide practical outward interface between the several machines and a host computer or a system for transfering wafers into and out of the handling system.
The systems now available also inherently require the use of intermediate containers or carriers. That is, wafers are removed from a cassette and put into an intermediate boat and the boat is then transferred onto a process container or carrier for actual processing. THe load stations thus tend to be fairly large and require movement of wafers outside of laminar flow environments.